Cryogenic refrigerator cycle

ABSTRACT

Where a cryogenic refrigerator cycle incorporates pistons operating between a plenum and the working volume and the plenum pressure is controlled to control the loading on the plenum side of the pistons, short time increased refrigerator capacity is obtained for cooldown by bleeding fluid from the plenum into the working volume.

United States Patent [1 1 3,640,082 Dehne 1 Feb. 8, 1972 [54] CRYOGENIC REFRIGERATOR CYCLE 3,078,683 2/1963 Dros ..62/6 [121 Axel Mm, 1m Angel, ii ii 31323 [73] Assignee: Hughes Aircraft Com Culver Ci Calif. ty Primary Examiner-William J. Wye

Attorney-James K. Haskell and Allen A. Dicke, Jr. [22] Filed: June 8, 1970 [21 Appl. No.: 44,303 [57] ABSTRACT Where a cryogenic refrigerator cycle incorporates pistons operating between a plenum and the working volume and the [52] U.S.Cl. ..62/6,4l7/439,4l7/522 plenum Pressure is controlled to comm] the loading on the [51] "lush 9/00 plenum side of the pistons, short time increased refrigerator 0f capacity is obtained for cooldown from [he plenum into the working volume. [56] 1 References Cited g 8 Claims, 6 Drawing Figures UNITED STATES PATENTS I V H 2,824,430 4/ 1971- Rinia .....62/6

BACKGROUND This invention is directed to a cryogenic refrigerator cycle which has particularly arranged for short duty cycle extra refrigeration capacity for cooldown.

The prior art teaches a number of refrigerator cycles which are adaptable to miniaturization for providing closed-cycle cooling to a zone that is to be cryogenically refrigerated. In 1816 Robert Stirling invented the Stirling cycle. In 1834 John Herschel suggested that this cycle could be used as a refrigerator. Equipment has been developed which permits its use in miniature cryogenic equipment. A Stirling cycle refrigerator of that nature is shown in Malaker, et al. U.S. Pat. No. 3,074,244. In this patent, particularly FIG. 4, it is clear that the crank chamber within housing 7 contains the same gas as the working fluid above the pistons. The plenum gas can be connected by a check valve to the low-pressure part of the cycle to reuse the leakage gas.

In 1959, Howard 0. McMahon and William E. Gifford of Arthur D. Little, Inc. reported the Gifford-McMahon refrigeration system, which was well suited for miniaturization. The double-volume refrigerator consists of a pressure cylinder closed at both ends, a displacer within the cylinder and a regenerator through which the fluid moves as the displacer moves the gas from one expansion space to the other at opposite ends of the cylinder. The displacer does zero net work in the ideal case of zero pressure drop in the regenerator. Gifford U.S. Pat. No. 2,966,035 discloses this system. Again, where the compressor and displacer are crank driven, the crankcase serves as a plenum volume for gas which leaked from the working fluid cycle.

The Solvay cycle predates and is similar to a Gifford-Mc- Mahon refrigeration system, but involves a single volume. The Solvay cycle is shown in German Pat. No. 39,280 dated May 16, 1887. Again, a plenum serves as a chamber which contains the gas which leaked out of the working fluid cycle.

Another cryogenic cycle of significance for miniaturized application, and suitable for the cycle modification of this invention which provides minimum weight in a machine running under optimum conditions on a continuous duty cycle, .and

which is equipped to operate under heavier duty conditions during cooldown for rapid cooldown, is shown in Cowen U.S. Pat. No. 3,379,026 This patent illustrates a machine operating on a modified Vuilleumier cycle. g

In miniaturized machinery wherein weight versus performance is critical, such as in aircraft and missile systems, rapid cooldown is also required. Prior systems obtained rapid cooldown by employing'twospeed motors. The mechanical equipment was operated at a higher speed during the cooldown stage. The increase in refrigerator speed provided a higher work output and thus is a greater refrigeration capacity, but did so at the complexity of requiring a power source which was capable of two-speed modes of operation.

The prior art includes Damsz U.S. Pat. No. 3,220,200 which illustrates the prior art method for decreasing cooldown time. Damsz teaches operating at a higher expansion ratio during cooldown, and then reduces the expansion ratio by connecting a volume for continuous duty operation, after cooldown. This system requires a larger compressor for operating the system, because of the increased volume connected to the system during continuous duty operation. Thus, Damsz designed a system which has adequate capacity for reduced cooldown time, and after cooldown reduces its refrigeration capacity by reducing its expansion ratio. Such a system does not lend itself to minimized weight for a particular cooling capacity in miniaturized machinery.

Cryogenic refrigerators, such as described above, are used for cooling infrared detectors or parametric amplifiers. Operational status is not achieved until the detector or amplifier is cooled to its proper temperature. In the case of defense devices, the time required to achieve operational status is often critically important.

SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a cryogenic refrigeration cycle wherein a plenum chamber on one side of a piston in a cryogenic refrigeration system contains the same fluid as the working fluid, and is employed to substantially equalize the average pressure differential across the pistons to minimize bearing loads during normal running. A valve connected from the plenum chamber to the working fluid system permits increased pressure during cooldown for maximum cooldown rate with no increase in machinery speed.

It is thus an object of this invention to provide a cryogenic refrigerator cycle which achieves rapid cooldown to quickly obtain operational temperature at the refrigerated point. It is another object to provide a cryogenic refrigerator cycle which obtains quick cooldown in a miniature refrigerator system which has a minimum of weight and complexity for the cooldown rate and cooling capacity. It is a further object to provide a cryogenic refrigerator cycle which employs singlespeed running during cooldown and continuous operation, but achieves rapid cooldown through increasing the amount of refrigeration fluid in the working cycle during the cooldown period. It is a further object to provide a cryogenic refrigerator cycle wherein a plenum chamber has fluid therein which reduces bearing loading by reducing pressure differentials across a cryogenic refrigerator piston, during normal operation, and employs a valve which connects the plenum chamber to the working cycle to permit additional working fluid in the working cycle during rapid cooldown. It is a further object to provide an interconnecting valve between the plenum chamber and the working fluid, which interconnecting valve controls the amount of refrigerant flow from the plenum space into the working fluid system.

Other objects and advantages of this invention will become apparent from the study of the following portion of the specification, the claims and the attached drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing of a Stirling cryogenic refrigeration cycle having incorporated therein the cycle modification of this invention.

FIG. 2 is a schematic showing of a Gifford-McMahon cryogenic refrigerator cycle having the cycle modification of this invention incorporated therein.

FIG. 3 is a schematic showing a Solvay cryogenic refrigerator cycle having the cycle modification of this invention incorporated therein.

FIG. 4 is a longitudinal section through a first embodiment of a valve assembly for employment in the cryogenic refrigeration cycle.

FIG. 5 is a longitudinal section, with some parts shown in side elevation, of a second embodiment of a valve assembly for use in the cryogenic refrigeration cycle of this invention.

FIG. 6 is a section through a third embodiment of the valve assembly for use in the cryogenic refrigeration cycle of this invention.

DESCRIPTION FIG. 1 illustrates a Stirling cycle cryogenic refrigerator modified in accordance with the cryogenic refrigerator cycle of this invention. The Stirling refrigerator cycle is a cycle of broad utility, and is not dependent on the specific machine illustrated in FIG. 1. However, the machine of FIG. 1 was designed to operate upon the Stirling cycle, and thus is a suitable vehicle for the explanation thereof. The Stirling refrigeration machine is generally indicated at 10. It has a crankshaft l2 rotating on an axis normal to the drawing. Crankshaft I2 is connected to a suitable drive motor for normal rotation at a constant speed. Crankshaft 12 carries an eccentric crankpin 14 to which are connected connecting rods 16 and I8. Compressor piston 20 operates in compressor cylinder 22 to define compressor working volume 24.

Similarly, expander piston 26 reciprocates in expander cylinder 28 to define expander working volume 30. Working volumes 24 and 30 are interconnected through regenerator 32, which is of conventional construction, such as regenerators 114 and 116 described below. The Stirling refrigeration machine includes a crankcase 34, in which the crankshaft is mounted for rotation, which encloses the connecting rods, and on which the cylinders are mounted. Thus, the crankcase encloses a plenum plenum 36.

The normal Stirling cycle operation is fully described in Malaker, et al. U.S. Pat. No. 3,074,244, and the machine just described operates in normal cycle operation during continuous duty operation. During continuous duty operation it is clear that working fluid, such as helium gas, escapes past the sealing rings on pistons 20 and 26 to pressurize plenum 36. The end result is that plenum 36 is pressurized about to the average pressure in the working volumes 24 and .30. One of the effects of this pressurization of the plenum is that the crank sides of the pistons 20 and 26 are pressurized to minimize bearingloads on the crankshaft, crankpin and wristpins in the pistons. For optimum miniaturization, this pressurization is considered in design so that the bearings are minimized in sizeand weight. Under these optimum conditions, the bearings thus carry substantially full load on a continuous duty cycle bearing rating.

In order tomaintain the plenum at a pressure above the lowest pressure, at the compressor suction in the cycle, a control valve between the plenum and the low-pressure side of the working part of the cycle is required. The plenum pressure cyclically varies because the plenum volume changes with piston motion. However, the volume of the plenum is large so that the compression ratio in the plenum is quite small. Thus, the plenum pressure can be considered to be constant at the median plenum pressure. When the valve is setat a constant pressure for the plenum the mean compressor pressure may be higher when the refrigerator is warm, and may be lower as the refrigerator cools down. This can also be true when a valve having a constant differential pressure capability is employed. A valve having a constant differential capability is preferred because this means that at the beginning of cooldown the mean plenum pressure is higher than in steady-state operation. This also aids in obtaining higher working pressures when such a differential pressure is maintained substantially constant.

The gross refrigeration capacity .of any refrigerator is defined by the displaced volume in the expander space, the speed of refrigeration operation and the pressure ratio in the refrigerated space. For any particular gas and refrigerator cycle, these criteria determine the gross refrigeration capacity of that refrigerator. Any deviations of the indicator card from the theoretical cycle diagram are reductions from the gross refrigeration capacity, but such deviations are expected in any practical cycle. Specific operating conditions for the Stirling and rectangular PV diagrams, of a specific example, are as follows:

Cycle with Rectangular PV Diagram such as Gifiord- In order to increase the cooldown rate without change in refrigerator speed or design conditions, valve assembly 38 permits the discharge of fluid from plenum 36 into the working volume. When the refrigerator starts up in the warm state, and is run a few revolutions to permit pressures to stabilize at the start of cooldown, and the working fluid is permitted to establish a balance between the amount in the plenum and the amount in the working volumes, the pressure range in working volume'30 is from 225 to 450, representing a refrigeration of 3.75 watts operating under otherwiseidentical conditions to those described for cold running.

This higher pressure operation necessarily results in higher bearing loadings. The cracking pressure of the valve is designed so that the resulting higher loadings are within the permissible dynamic load limits of the bearings so that the loads can be tolerated for the short time which they are endured during cooldown period.

Three examples of suitable devices for employment as the valve assembly 38 are illustrated in FIGS, 4, 5 and 6. Valve assembly 40 is illustrated in FIG. 4. It comprises a body 42 which is connected by line 44 to the refrigeration machine plenum and by a line 46 to the working volume of the refrigerator. Body 42 contains seat 48 against which a valving member such as ball 50 can engage.

Bimetallic spring 52 is seated in spring holder 54 and is engaged against ball 50. Spring holder 54 is threadably engaged in body 42 so that the resilient loading on ball 50 can be adjusted. Line 46 is mounted on plug 56 which is screwed into the body so that line 46 receives all of the fluid passed by valve ball 50. Bimetallic spring 52 acts both as a mechanical spring and as a bimetallic element. During any one temperature operation it controls the pressure differential between the lowest pressure in the working cycle and the plenum. lts physical temperature controls this differential. In accordance with this invention, at lower temperatures the bimetallic spring 52 has less stress and permits lower pressure differentials between the plenum and the low-pressure part of the cycle to obtain a faster refrigeration rate than in normal continuous duty operation, as is described above.

In FIG. 1, valve assembly 38 is illustrated as being secured to the side of the compressor cylinder. In this location, the valve body is at ambient temperature until the refrigeration machinery is started. The lower temperature situation, described above, is that which is operative during startup period. The heat of compression after the refrigerator is started causes warmup of the compressor cylinder 22, and the valve body lying against it. The thermal connection is arranged in such a manner that the warmup of the valve body is approximately the same as the rate of cooldown in the expansion cylinder. Thus, as the machine runs, it resultsin a higher temperature of the valve body. This higher temperature is a product of the temperature stabilization of the crankcase after startup and results from the heat of compression of the refrigerant. By controlling the thermal gradient from the compression chamber to the valve, one can effectively design a lag time between startup temperature and final operational temperature stabilization equal to the required cooldown time.

FIG. 5 is directed to a valve assembly 58 which can be remotely controlled. Valve assembly 58 has a body 60 to which lines 62 and 64 are connected which respectively are connectable to the plenum and working cycle of a refrigera-' tion machine. Body 60 contains seat 66 against which valve disc 68 is urged by means of spring 70. The other end of spring 70 is engaged by magnetic core 72 which is reciprocable within body 60. Coil 74 embraces the body. Coil 74 is connectable so that upon varying energization, it acts upon core 72 to variable load spring 70. Thus, by adjustment of the energization of coil 74, the maximum differential pressure between lines 62 and 64 is controlled.

FIG. 6 illustrates a valve assembly 174 which has a body 176. The body 176 has an inlet passage at the bottom and an outlet passage at the top which are respectively connected to the refrigeration apparatus plenum and a point in the lowpressure part of the cycle. Valve ball 178 restrains flow from the plenum to the low-pressure side.

Temperature sensor 180 is positioned at the refrigeration point on the machine, and thus is responsive to temperature at the cold point, and can determine whether or not the refrigeration apparatus has cooled down the cold basis. Temperature sensor 180 can be a vapor-filled device in which pressure decreases as the temperature goes down. Sensor 180 is connected to bellows 182 which is in turn connected to the ball 178 through spring lever 184. The spring lever passes over a fulcrum so that as temperature decreases at the cold point, the valve ball 178 is urged more firmly into its seat to maintain a greater pressure differential between the plenum and the low-pressure point of the cycle. Thus, plenum pressure builds up to decrease working loading on the bearings which drive the pistons. Thus, the structure of FIG. 6 is also a valve assembly which can be employed to control plenum pressure in accordance with this invention, both with respect to an increased cooldown rate and with respect to decreased bearing loading during steady-state operation.

A Gifford-McMahon refrigeration machine is illustrated generally at 76 in FIG. 2. The refrigeration machine has a compressor 78 which is formed of a cylinder 80 in which piston 82 reciprocates. This defines compressor working volume 84 and plenum 86. Crankshaft 88 has a crankpin 90 which is connected by connecting rod 92 to the piston. As the crankshaft rotates, piston 82 reciprocates in the cylinder to change the volume of compressor working volume 84. Lowpressure tank 94 is connected through check valve 96 to the working volume 84, to deliver working fluid to the working volume. Similarly, high-pressure tank 98 is connected by check valve 100 to receive compressed working fluid from the compressor working volume 84. Thus, compressor 78 serves to receive working fluid from the low-pressure tank 94 and deliver it at higher pressure to high-pressure tank 98. In larger size equipment the compressor 78 might be replaced by rotating machinery, such as a vane compressor, but in the small size units for which this invention is particularly suited, the reciprocating compressor 78 illustrated is preferred.

Expander cylinder 102 has inlet and exhaust valves 104 and 106 connected thereto to supply and receive working fluid from the lower working volume 108 therein. Displacer 110 reciprocates within cylinder 102 to define the lower working volume and the upper working volume 112. The interior of displacer 110 contains regenerator 114. The regenerator is open to both the top and the bottom of the displacer to the working volumes. The regenerator is conventionally made of punched metal sheets or screens, spaced with low heat transfer separators. For example, punched copper or lead sheets can be separated by stainless steel wire coils. As the displacer 110 reciprocates, it moves working fluid back and forth between the volumes 108 and 112 through the regenerator. The pressure at which the working volume is operating at a particular point in the cycle is accomplished by control of valves 104 and 106.

Crankshaft 116 is rotatably mounted in plenum 118. Crankshaft 116 may conveniently be attached to rotate with crankshaft 88, and is preferably so arranged in a smaller refrigeration machine. However, in larger machines they are conventionally separately driven. A camshaft rotating with crankshaft 116 conveniently operates valves 104 and 106. Crankshaft 116 carries crankpin 120 which is connected to displacer 110 by means of connecting rod 122. Thus, by rotation of the crankshaft 116, the displacer reciprocates and the valves 104 and 106 are operated. The system produces refrigeration in accordance with the cycle described in Gifford U.S. Pat. No. 2,966,035.

In view of the essentially balanced forces on the end of displacer 110, pressurization of the expansion chamber 112 does not produce high connecting rod and crankshaft bearing loads. Consequently, the effect of pressurization of plenum 118 has minimum quantitative effect on bearing loading due to the small diameter of piston rod 124 which extends from the displacer through lower working volume 108 into plenum 118. Thus, plenum 118 collects working fluid which escapes past the seal around the piston rod until all pressures equalize.

In order to limit the back pressure in plenum 86 to the maximum desirable value, valve assembly 128 is connected between the plenum 86 and low-pressure tank 94. Valve assembly 128 is in the form of a relief valve which, during normal continuous operation of the Gifford-McMahon refrigeration machine 76, limits the maximum pressure in plenum 86 to substantially equal to the average pressure in compressor working volume 84.

The valve assembly 128 can be any of the valve assemblies 40, 58 or 174. With these valve assemblies, during startup when rapid cooldown is desired, additional gas is discharged from the plenum volumes into the working volume part of the cycle so that the pressure is raised in the working volume to cause more refrigeration per unit time. The gas moving through valve 128 aids in maintaining pressure during cooldown to obtain maximum cooldown rate which is consistent with maximum dynamic bearing loading for short time operation in this specific machine. The increase in gas in the working cycle increases pressure to range from a low pressure of 225 to a high pressure of 450. This results in a 50 percent increase in refrigeration capacity. The operating criteria of the Gifford-McMahon refrigeration machine are selected to be similar to those of the Stirling cycle machine described above. The pressures involved and the enhanced performance upon cooldown of this cycle are substantially the same as that for the Stirling cycle.

Referring to FIG. 3, the Solvay cycle refrigeration machine is generally indicated therein at 130; Compressor 132 is identical to the compressor 78, including the compressor working volume 134 and plenum 136 separated by compressor piston 138.

Similarly, the compressor 132 draws working fluid from low-pressure tank 140 through check valve 142 and delivers high-pressure fluid through check valve 144 to high-pressure tank 146. Expander cylinder 148 has a displacer 150 therein. The displacer separates expander cylinder into working volume 152 and plenum 154. Crankshaft 156 has a crankpin 158 which is connected by connecting rod 160 to displacer 150. Thus, by rotation of crankshaft 156, displacer I50 reciprocates within cylinder 148. Regenerator 162 is formed within the displacer, for such is the preferred and most efficient structure for miniaturized refrigerators. However, it could be externally located at the cost of greater thermal losses. Regenerator 162 is open at its top to working volume 152, and is open at its bottom through a sliding fluid joint to line 164. Valve 166 and 168 are respectively connected to tanks 146 and 140, and to line 164. Valves 166 and 168 are controlled to control fluid into and out of regenerator 162 in accordance with cycle requirements. The valves are preferably controlled by cams which are operated in response to angular position of crankshaft 156. The actual refrigeration cycle of the Solvay refrigeration machine 130 is described in more detail in Cryogenic Systems" by Randall Barron, Mc- Graw-Hill 1966, pages 323-326, the entire disclosure of which is incorporated herein by this reference. Although that publication credits the type of refrigeration cycle shown in FIG. 3 to Gifford and McMahon, it was earlier shown in the German Pat. No. 39,260 dated May 16, 1887 to Solvay. The Gifford- McMahon refrigeration machine in FIG. 2 is described in detail on that reference at pages 326-33 l.

The working fluid leakage downward past displacer 150 is received in plenum 154. Pressure in plenum 154'stabilizes at the mean pressure in expansion space 152. Thus, load is equalized as much as possible between the two ends of expansion piston 150.

Leakage past piston 138 in the compressor goes to the compressor plenum 136. Compressor plenum 136 is connected through valve assembly 172 to the low-pressure part of the refrigeration cycle. Valve assembly 172 may be a constant differential pressure device "it; maintain constant differential pressure in the plenum with respect to the low-pressure tank, and to relieve excessive pressure to low-pressure tank 140, it could be a constant pressure valve, or it can be any one of the valve assemblies as shown in FIG. 4, and 6. These are responsive to temperature and to external control so that by this means the plenum pressure is adjusted in plenum 136 to be the mean of high and low pressures (or average pressures) on the working side of the piston, for steady-state operation.

For maximum performance of the refrigeration machine during startup and cooldown, the valve assemblies of FIGS. 4, 5 and 6 are operated to bleed the fluid from the plenums into the low-pressure portion of the system in which the working fluid is employed to produce refrigeration. The amount of working fluid transferred can be up to a maximum of a working fluid load which brings the bearing loads to their maximum dynamic short term loading. Such an increase of fluid increases the working pressure to the expansion range of 225 to 450 p.s.i. This results in a gross refrigeration increase of 50 percent over the steady-state operation. The operating criteria of the Solvay refrigeration machine are selected to be similar to those of the Stirling cycle machine described above. The pressures involved and the enhanced performance upon cooldown of this cycle are substantially the same as that for the Stirling cycle.

This invention having been disclosed in its preferred embodiments, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the invented faculty.

What is claimed is:

l. A cryogenic refrigeration machine, said cryogenic refrigeration machine comprising:

a cylinder;

a piston reciprocable within said cylinder to divide said cylinder into a working volume and a plenum;

mechanical means interconnected between said piston and said cylinder for controlling the reciprocable motion of said piston within said cylinder so that the pressure of working fluid in said working volume varies, the improvement comprising:

working fluid flow control means interconnected between said plenum and said working volume, said working fluid flow control means comprising a pressure control valve having a higher pressure inlet side and a lower pressure outlet side, said higher pressure inlet side being connected to said plenum and said lower pressure outlet side being connected to said working volume for maintaining the working fluid pressure within said plenum above the minimum working fluid pressure within said working volume during normal refrigeration operation.

2. The cryogenic refrigeration machine of claim 1 wherein said mechanical means between said piston and said cylinder is a crankshaft rotatably mounted with respect to said cylinder and a connecting rod interconnecting said piston and said crankshaft, so that the working fluid pressure in said plenum maintained by said working fluid flow control means reduces loading on said crankshaft and said connecting rod.

3. The cryogenic refrigeration machine of claim, 2 wherein said working fluid flow control means is a valve and wherein valve control means is connected to said valve to permit flow of fluid from said plenum into said working fluid chamber when pressure in the said plenum exceeds the value controlled by said control means.

4. A cryogenic refrigeration machine, said cryogenic,

within said plenum, for controlling reciprocable motion of said piston within said cylinder so that the pressure of working fluid in said working volume varies, the improvement comprising;

a valve connected between said plenum and said working volume, and valve control means connected to said valve said valve comprising a valve member on a seat and said valve control means comprising thermostatically controlled spring means urging said valve member against its seat, said thermostatically controlled spring means having a lower spring force at higher temperatures and a higher spring force at lower temperatures for maintaining the working fluid pressure within said plenum above the minimum working fluid pressure within said working volume for reducing loading on said crankshaft and said connecting rod.

5. The cryogenic refrigeration machine of claim 4 wherein said control means comprises a bimetallic spring urging said valve member against its seat.

6. The cryogenic refrigeration machine of claim 3 wherein said valve control means comprises electromagnetic control means having a coil and having a magnetic core within said coil, said magnetic core urging said valve member against its seat so that upon coil energization said valve member is urged against itsseat.

7. The cryogenic refrigeration machine of claim 6 wherein a spring is interpositioned between said magnetic core and said valve member so that energization of said coil loads said spring and spring loading urges said valve member against its seat with a force related to coil energization.

8. The process of operating a cryogenic refrigeration machine which has a cylinder, a piston reciprocable within the cylinder to divide the cylinder into a working volume and a plenum, a connecting rod in the plenum to reciprocate the piston within the cylinder to cause pressure changes in cryogenic refrigeration fluid in the working volume and the plenum, and cryogenic working fluid flow control means interconnected between the plenum and the working volume comprising the steps of:

upon initial startup of the cryogenic refrigeration machine,

increasing the amount of working fluid in the working volume by permitting the working fluid flow control means to pass working fluid from the plenum to the working volume to increase the maximum pressure differential between the working volume and the plenum toward a point where the connecting rod bearings are at their maximum dynamic short term loading; and

subsequently, after cooldown of the cold portion of the cryogenic working fluid, operating the working fluid control means to decrease the maximum pressure differential between the working volume and the plenum to a value which reduces the bearing loading below the bearing maximum dynamic short term loading so that maximum rapid cooldown is obtained for a particular cryogenic refrigeration machine. 

1. A cryogenic refrigeration machine, said cryogenic refrigeration machine comprising: a cylinder; a piston reciprocable within said cylinder to divide said cylinder into a working volume and a plenum; mechanical means interconnected between said piston and said cylinder for controlling the reciprocable motion of said piston within said cylinder so that the pressure of working fluid in said working volume varies, the improvement comprising: working fluid flow control means interconnected between said plenum and said working volume, said working fluid flow control means comprising a pressure control valve having a higher pressure inlet side and a lower pressure outlet side, said higher pressure inlet side being connected to said plenum and said lower pressure outlet side being connected to said working volume for maintaining the working fluid pressure within said plenum above the minimum working fluid pressure within said working volume during normal refrigeration operation.
 2. The cryogenic refrigeration machine of claim 1 wherein said mechanical means between said piston and said cylinder is a crankshaft rotatably mounted with respect to said cylinder and a connecting rod interconnecting said piston and said crankshaft, so that the working fluid pressure in said plenum maintained by said working fluid flow control means reduces loading on said crankshaft and said connecting rod.
 3. The cryogenic refrigeration machine of claim 2 wherein said working fluid flow control means is a valve and wherein valve control means is connected to said valve to permit flow of fluid from said plenum into said working fluid chamber when pressure in the said plenum exceeds the value controlled by said control means.
 4. A cryogenic refrigeration machine, said cryogenic refrigeration machine comprising: a cylinder: A piston reciprocable within said cylinder to divide said cylinder into a working volume and a plenum; A crankshaft rotatably mounted with respect to said cylinder and a connecting rod interconnecting said piston and said crankshaft, said connecting rod being positioned within said plenum, for controlling reciprocable motion of said piston within said cylinder so that the pressure of working fluid in said working volume varies, the improvement comprising; a valve connected between said plenum and said working volume, and valve control means connected to said valve said valve comprising a valve member on a seat and said valve control means comprising thermostatically controlled spring means urging said valve member against its seat, said thermostatically controlled spring means having a lower spring force at higher temperatures and a higher spring force at lower temperatures for maintaining the working fluid pressure within said plenum above the minimum working fluid pressure within said working volume for reducing loading on said crankshaft and said connecting rod.
 5. The cryogenic refrigeration machine of claim 4 wherein said control means comprises a bimetallic spring urging said valve member against its seat.
 6. The cryogenic refrigeration machine of claim 3 wherein said valve control means comprises electromagnetic control means having a coil and having a magnetic core within said coil, said magnetic core urging said valve member against its seat so that upon coil energization said valve member is urged against its seat.
 7. The cryogenic refrigeration machine of claim 6 wherein a spring is interpositioned between said magnetic core and said valve member so that energization of said coil loads said spring and spring loading urges said valve member against its seat with a force related to coil energization.
 8. The process of operating a cryogenic refrigeration machine which has a cylinder, a piston reciprocable within the cylinder to divide the cylinder into a working volume and a plenum, a connecting rod in the plenum to reciprocate the piston within the cylinder to cause pressure changes in cryogenic refrigeration fluid in the working volume and the plenum, and cryogenic working fluid flow control means interconnected between the plenum and the working volume comprising the steps of: upon initial startup of the cryogenic refrigeration machine, increasing the amount of working fluid in the working volume by permitting the working fluid flow control means to pasS working fluid from the plenum to the working volume to increase the maximum pressure differential between the working volume and the plenum toward a point where the connecting rod bearings are at their maximum dynamic short term loading; and subsequently, after cooldown of the cold portion of the cryogenic working fluid, operating the working fluid control means to decrease the maximum pressure differential between the working volume and the plenum to a value which reduces the bearing loading below the bearing maximum dynamic short term loading so that maximum rapid cooldown is obtained for a particular cryogenic refrigeration machine. 